Electrically powered space heaters that blow hot air out into a space to be heated are well known in the art. Many employ an electrically resistive element that is heated by an electric current. Air forced over the hot element is heated. The very hot element may be a safety hazard. Heaters that generate heat by friction of rotating discs rubbing against stationary discs are exemplified by U.S. Pat. No. 4,295,461 issued 10/20/81 to Cummings et al. Problems encountered with such devices include efficient heat transfer from the hot discs to a circulating heat transfer fluid as disclosed in U.S. Pat. No. 3,402,702 issued 9/24/68 to Love and wear of frictional surfaces as disclosed in U.S. Pat. No. 4,524,755 issued 6/25/85 to Harris et al and also U.S. Pat. No. 2,090,873 issued 8/24/37 to Lazarus in which it is disclosed that lignum vitae rubbing against cast iron has suitable wear resistant properties. Unfortunately using lignum vitae and cast iron for their wear resistant properties complicates heat transfer problems because these materials have a poor thermal conductivity and must be made thick because they are weak in thin section. And blocks of material that move up to replace worn away surface are also excessively thick for efficient heat transfer.